1. Field of the Invention
This invention relates to a method of producing single crystal fibers for use as infrared (IR) transmissive waveguides and in various nonlinear optical interactions. The single crystal fiber is produced from congruently melting polycrystalline fiber feedstock via a continuous process. Variations within the process permit production of a fiber with a polycrystalline core surrounded by a large grained or single crystal sleeve, as well as surface improvement of an already existing single crystal fiber.
2. Description of the Prior Art
Single crystal fiber growth has been obtained using a number of different methods. The earliest and most basic is the Stepanov method, a derivative of the Czochralski method of bulk single crystal growth, in which the feedstock is melted in an inert crucible and held at melting point while a crystal is pulled from the free surface of the melt. The shape of the crystal is influenced by a crucible orifice, functioning as a shaper, with dimensions approximating those of the crystal. There are numerous variations within this method, including the La Belle, Bridges, and Mimura techniques for shaped crystal growth which utilize a shaper, the surface of which physically contacts the melt in a manner which shapes the crystal through surface tension of the liquid. The Stepanov techniques are described by Antonov, P. I. et al., "Physical Problems in Crystal Growth by the Stepanov Method", Journal of Crystal Growth, 50, 3-7 (1980) and La Belle, H. E. Jr. et al., "EFG, The Invention and the Application to Sapphire Growth", Journal of Crystal Growth, 50, 8-17 (1980). The length of the fiber obtained using the Stepanov method is determined by the diameter of the fiber and the volume of melt in the crucible.
Methods have also been developed for continuous growth of single crystal fibers. One method of continuous growth of single crystal fibers is described by Y. Mimura et al in "Growth of Fiber Crystals for Infrared Optical Waveguides", Japanese Journal of Applied Physics, 19, L269-L272 (1980). This method uses a polycrystalline feed rod which is melted in a crucible and flows through a heated capillary to a heated shaper via gravity. Upon exiting the shaper in a downward direction, the melt is frozen into a single crystal fiber which is transported between rollers to a collection area. The capillary and shaper are used to confine and shape the melt into the resultant fiber.
Another method of growing single crystal fibers from a melt is described by T. J. Bridges et al. in "Single-Crystal AgBr Infrared Optical Fibers", Optics Letters 5, 85-86 (1979). This method differs from that of Y. Mimura et al. in that the feed to the capillary and shaper is controlled using nitrogen gas pressure on the melt crucible, the fiber is pulled upward from the melt, and the total amount of feedstock is that contained in the melt crucible, initially, before nitrogen gas pressure is applied.
A method for continuous growth of polycrystalline fibers has also been developed. Polycrystalline fibers for use in infrared radiation transmission have been continuously extruded using elevated temperatures and high pressures. This process is described in a patent application of D. A. Pinnow et al., Ser. No. 230,923 filed on Feb. 2, 1981, now U.S. Pat. No. 4,451,116 and assigned to the assignees of the present application. Application Ser. No. 230,923 is a continuation of application Ser. No. 037,581, filed May 9, 1979, which was a continuation of the parent application, Ser. No. 800,149, filed May 24, 1977, now abandoned.
Single crystal silicon rods for use in the semiconductor industry are also grown by a continuous process. Typically, a polycrystalline feed rod is melted and recrystallized to convert it from polycrystalline form to a single crystal. However, the diameter of such feed rods ranges from about one centimeter to ten centimeters, and thus a special seeding technique is required to insure single crystal growth. In addition, in order to obtain cylindrical crystals, the feed rod or at least the seed crystal is rotated, and the melt is achieved using an induction heater which generates an electromagnetic field which assists in shaping during the recrystallization process. This continuous process is known as the "float" zone method because the melt is not confined by a capillary or shaping tube. It is important to emphasize that the linear growth rate of these single crystal semiconductor rods decreases as the diameter of the rods increases. The length of melt zone necessary to achieve complete melting of the cross-section of the rods described above results in some flow of the melt, such that a mechanical equilibrium exists. Since the resultant equilibrium is mechanical, a steady state condition does not exist, and the crystal diameter varies periodically as is manifested by ripples on the surface of the rod, such ripples having an amplitude exceeding the typical diameter of a fiber. Recent patents which provide a good description of the float zone method are U.S. Pat. Nos. 4,258,009, 4,257,841, and 4,220,839.
The prior art, as typified in the Stepanov method, has demonstrated that single crystal fibers for use in infrared (IR) transmission and nonlinear electro-optical devices can be drawn from the free surface of a melt contained in a crucible or confined by a shaper. However, such fibers frequently exhibit undesirable surface characteristics in the form of faceting or presence of contaminants. The faceting is due to both thermal gradients unavoidable in the growth process, and strain which is induced when shapers or capillary tubes are used to control the dimensions of the fiber as it is drawn; the contaminants often arise from contact with the crucible or shaper. The surface characteristics described above result in loss of IR transmission due to scattering and absorption.
The prior art has also demonstrated that polycrystalline fibers for use in IR transmission can be fabricated using pressure driven extrusion techniques. However, such polycrystalline fibers cannot be used in nonlinear optical devices which require the lattice structure of a single crystal. In addition, residual strains in the fibers induced during the extrusion process are believed to contribute significantly to transmission losses.
It is therefore desirable to provide a method which will permit conversion of IR transmitting polycrystalline fibers to single crystal fibers which are optically isotropic in the case of transmitting materials or optically anisotropic in the case of nonlinear materials, and free from surface defects and residual strain.